The invention relates to a method for setting a detection threshold with which a received signal of a frequency-modulated continuous-wave radar sensor of a motor vehicle is compared method with the aim of detecting a target object in the environment of the motor vehicle. In successive measuring cycles the radar sensor emits in each case a radar signal into a capture zone and receives a radar signal reflected within the capture zone as a received signal. In each measuring cycle a frequency spectrum relating to the respective received signal is determined, the individual frequency bins of which frequency spectrum correspond respectively to a signal level, in particular to the power of the received signal, in a range-resolution cell of the radar sensor. In each measuring cycle the detection threshold is set individually in each case for a subset consisting of at least one frequency bin, and in order to set the detection threshold for the respectively current subset of the frequency bins a noise level of the frequency spectrum is determined. The detection threshold is then set as a function of the noise level, for example by multiplication of the noise level by a predetermined factor. The invention relates, in addition, to a frequency-modulated continuous-wave radar sensor for a motor vehicle, and also to a motor vehicle with such a radar sensor.
Radar sensors for motor vehicles (automotive radar sensors) are already state of the art and are operated, for example, at a frequency of about 24 GHz or about 79 GHz. Radar sensors serve generally for the detection of target objects in the environment of the motor vehicle, and assist the driver in numerous respects in connection with the driving of the motor vehicle. In the present case, the focus is on, in particular, towards a blind-spot detection system (blind-spot warning), by means of which the driver is warned of the presence of target objects in the blind-spot zone of the motor vehicle.
Radar sensors measure, on the one hand, the spacing between the target object and the vehicle. They also measure, on the other hand, both the relative velocity with respect to the target object and also the so-called target angle, i.e. an angle between an imaginary connecting line leading to the target object and a reference line, for instance the longitudinal axis of the vehicle or a radar axis extending perpendicularly with respect to the front face of the sensor. With the aid of a radar sensor, the respectively current position of the target object relative to the vehicle can consequently be determined, and the target object can be tracked in the capture zone of the radar sensor, i.e. the relative position of the target object can be determined continuously over a plurality of measuring cycles of the radar sensor. Tracking is possible on the assumption that the points of reflection detected on the target object remain stable over the measuring cycles.
Radar sensors are usually placed behind the bumper, for example in the respective corner regions of the rear bumper. In order to detect the target object, the radar sensor emits a transmitted signal (electromagnetic waves) which is then reflected at the target object to be detected and is received as a radar echo by the radar sensor. In the present case, it is a question in particular of the so-called frequency-modulated continuous-wave radar sensor (FMCW radar), in which the emitted signal comprises a sequence (burst) of frequency-modulated chirp signals which are emitted one after the other. Correspondingly, the received signal of the radar sensor also comprises such a plurality of chirp signals which are processed and evaluated with regard to the aforementioned measured variables. In general, at least one chirp signal can be emitted. The received signal in this case is firstly downmixed into the baseband and subsequently converted, by means of an analogue-to-digital converter, into a digital received signal with a plurality of sampled values, and subjected to an FFT (fast Fourier transformation). The sampled values are then processed by means of an electronic computing device (digital signal processor) in the time domain and/or in the frequency domain.
With a radar sensor, a relatively wide azimuthal angular range, which may even amount to 150°, is typically captured in the horizontal direction. The radar sensor therefore exhibits a relatively large azimuthal capture angle, with the result that the field of view or the capture zone of the radar sensor in the azimuth direction is correspondingly wide. The azimuthal capture angle is, as a rule, symmetrical with respect to a radar axis extending perpendicularly with respect to the front face of the sensor, with the result that the azimuthal capture angle of, for example, −75° to +75° is measured with respect to the radar axis. This azimuthal capture zone can be subdivided into smaller subzones which are irradiated and captured, one after the other, by the radar sensor. For this purpose, the main lobe of the transmitting antenna is, for example, swivelled electronically in the azimuth direction, for example in accordance with the phase-array principle. The receiving antenna may in this case exhibit in the azimuth direction a receiving characteristic with which the entire azimuthal capture zone is covered. Such a radar sensor is known, for example, from document DE 10 2009 057 191 A1. A further radar sensor is known from document US 2011/0163909.
Accordingly, the blind-spot zone of a motor vehicle can also be monitored with the aid of a radar sensor, and the driver can be warned where appropriate. In the state of the art, the functionality of blind-spot monitoring is based on the stated tracking of a target: the radar sensor firstly detects the target object, for example another vehicle, and tracks this target object in the capture zone. If the target object enters—for example when overtaking—a predetermined warning zone that corresponds to the blind-spot zone, a warning signal is output in the motor vehicle. The driver is consequently informed of the presence of the target object in the blind-spot zone. In order to be able to track the target object over a plurality of measuring cycles of the radar sensor, it is necessary to obtain a sufficient number of rough detections relating to one and the same target object. This means that a point of reflection, detected in a particular measuring cycle, of the target object also has to be detected in a subsequent measuring cycle. Accordingly, stable tracking of the target object means, in other words, that the detections originate in each case from a point of reflection of the target object that remains stable in its range and angle between successive measuring cycles. Consequently, points of reflection which are detected in differing measuring cycles are assigned to one another.
The detection of points of reflection alone is typically effected in the frequency domain. For this purpose the received signal is subjected to a Fourier transformation, namely the FFT (fast Fourier transformation), and a frequency spectrum of the already downmixed received signal is determined. The points of reflection of target objects are represented by peaks in the received spectrum. In the course of the FFT, power values of the received signal are determined for a plurality of frequency values (and, more precisely, of small frequency intervals). These power values are also known by the designation ‘FFT bin’ or ‘frequency bin’. The FFT accordingly provides a plurality of frequency bins which respectively represent a signal value for a particular frequency domain, or, a particular frequency value. Because in the case of an FMCW radar the frequency of the received signal also depends on the range of the target object, each frequency bin is respectively assigned to a range-resolution cell. The size of a range-resolution cell defines here the resolution of the radar sensor, with which the range of the target object can be determined.
The target echoes and the measuring noise are superimposed additively. The measuring noise is frequency-dependent and increases at higher frequencies. Also by virtue of so-called ‘clutter’—that is to say, undesired reflections on the ground, vegetation and extensive infrastructure objects—additional interference signals are superimposed on the useful signal in a frequency-dependent manner. In order to enable the detection of a target echo in the received signal, the frequency bins—that is to say, the signal values—are compared with a detection threshold. The signal peaks or target echoes that are present in the received signal are detected with the aid of a threshold-value detector. If the level of the frequency bin currently being examined lies above the detection threshold, detection takes place in the assigned range-resolution cell. The detection threshold is set adaptively during the operation of the radar sensor, specifically, for example, in accordance with the CFAR method (constant false-alarm rate). According to this method, the detection threshold is adapted in such a manner that it always lies above the noise level by a predetermined factor. If no target object is present, the detector incorrectly detects a target whenever the noise signal or interference signal lies above the detection threshold. If the detection threshold is set relative to the interference power, a constant false-alarm rate results, i.e. a constant probability that the instantaneous interference signal exceeds the detection threshold.
The detection threshold is calculated individually for each frequency bin. For this purpose, the CA-CFAR method, for example, may be applied, in which for each frequency bin a noise level is used for setting the detection threshold, said noise level being averaged arithmetically from the adjacent frequency bins. Another method (OS-CFAR) is to employ the so-called rank-order filter, in which a fixed rank is selected as the noise level. A rank-order filter of such a type has been shown in a schematic representation in FIG. 1. The FFT algorithm provides the frequency bins R which are respectively assigned to a range-resolution cell C0 to CN-1. The cell under test CUT will be examined here. The frequency bin R of this cell CUT is supplied to a comparator 100 to which, on the other hand, the detection threshold T that is determined individually for the CUT is also supplied. In order to determine the detection threshold T, the rank-order filter 200 orders the frequency bins R0 to RN-1 by size. A predetermined rank, namely frequency bin Rrk, is then selected as the noise level for the CUT and is multiplied by a predetermined factor C. The result constitutes the current detection threshold T. Frequency bin Rrk is accordingly used as the noise level that constitutes the basis for the establishment of the detection threshold T for the CUT. The frequency bins R from the range-resolution cells 300 immediately adjacent to the CUT are not taken into account in the determination of the noise level Rrk. These cells 300 constitute so-called ‘guard cells’. This is because a target echo that is present in the CUT typically extends also to the immediately adjacent cells 300, so that these cells 300 are not to be used for determining the noise level.
The noise level Rrk is therefore estimated in the state of the art by filtering the frequency bins over the range or over the range-resolution cells of an individual measuring cycle. Such a procedure has proved to be disadvantageous, particularly for a near-range zone: as already stated, by reason of the properties of the sensor and of the radar measurement, in particular by reason of the range-dependent and therefore frequency-dependent amplification of the received signal, the noise power in the near-range zone (in the first front range-resolution cells) is significantly lower than in the case of relatively long ranges or in the other range cells. If the frequency bins from the other range-resolution cells are also included in the estimation of the noise level for the first range-resolution cells, an elevated detection threshold results correspondingly for the near-range zone. This, in turn, leads to impaired sensitivity of the radar sensor in the near zone in comparison with relatively long ranges. This sensitivity is to be improved, particularly in the first five range-resolution cells. By reason of the reduced sensitivity of the radar sensor in the near zone, a brief loss of function may, in fact, occur in the course of the monitoring of the blind-spot zone. An already activated alarm may, for example, be briefly interrupted, even though a target object continues to be located in the blind-spot zone. This applies, in particular, to large lorries without an underride guard, in which no point of reflection is present, or only weak points of reflection are present, in the central longitudinal region of the lorry.
And conversely, if the frequency bins from the near range cells are used for determining the threshold for the other range cells, the threshold may be set too low, and consequently apparent echoes are detected.
It is an object of the invention to demonstrate a solution as to how, in the case of a method of the type mentioned in the introduction, the sensitivity of the radar sensor, particularly within a predetermined near-range zone, can be improved in comparison with the state of the art.
This object is achieved, according to the invention, by a method, by a radar sensor and also by a motor vehicle having the features according to the respective independent claims. Advantageous embodiments of the invention are the subject-matter of the dependent claims, the description and the figures.